1. Field of the Invention
The present invention relates to an image processing apparatus which converts image data into printing data for a printing apparatus by using color separation data and outputs the printing data, and a method thereof.
2. Description of the Related Art
Various types of image forming apparatuses used for word processors, personal computers, facsimile apparatuses, and the like are available. An apparatus which prints a text and image by attaching a printing material onto a printing medium has been practically used. A representative example of this printing type is an inkjet printer. The inkjet printer presents halftone by spatial arrangements of ink dots discharged from the printhead. In a region with a low dot density, i.e., highlight portion, the printed dots are visually noticeable, thereby enhancing graininess.
As described in Japanese Patent Laid-Open No. 2002-059571, a technique of making graininess unnoticeable in a highlight portion is available, in which light color materials such as Lc (Light cyan) and Lm (Light magenta) are used in addition to the four colors (CMYK) of the conventional basic color materials.
With the rapid prevalence of color inkjet printers in recent years, a demand for a high-quality image has arisen. Particularly, there is a demand to print a sharper image by expanding a color gamut reproducible by a printer. Upon expanding the color gamut, a color region reproducible by one of three colors C, M, and Y can be expanded by improving the color development properties or density of the color material. However, the conventional four colors CMYK can form only a limited color gamut, particularly for a color region with high chroma of R, G, and B regions formed by the color materials of two or more colors. Therefore, Japanese Patent Laid-Open No. 2001-136401 discloses a method of forming an image by adding a high-chroma color other than the subtractive primaries, i.e., specific color ink in addition to the basic color materials of four colors CMYK upon expanding the color gamut.
On the other hand, along with the improvement of image quality in color inkjet printers, printing digital images using the color inkjet printer, i.e., so-called photographic printing is becoming increasingly popular. As such photographic printing becomes popular, the storage life of a print is becoming regarded as important. Conventionally, dye ink has been mainly used as a color material for an inkjet printer. However, since the dye ink has poor storage properties, pigment ink with good storage properties is sometimes used. It is known that when the pigment ink is used in printing, a bronzing color unlike the original color of illumination is seen in the illumination image reflected on the surface of the print. This phenomenon is called bronzing. In order to suppress the occurrence of bronzing, Japanese Patent Laid-Open No. 2001-138555 proposes a method of determining color separation which suppresses bronzing by visually setting a patch which generates the least bronzing for each injection ratio from a patch image group obtained by changing the black level and UCR (Under Color Removal) ratio. A phenomenon called uneven bronzing in which bronzing that shows different tints in the different chromaticities is seen is also known.
As described above, with an increase in the number of color materials used in a color inkjet printer in recent years, when the color materials of two or more colors are used to reproduce a desired color, the number of combinations of the color materials increases. The degree of the bronzing viewed in a printed image differs depending on the combination of color materials. In order to select an optimum combination from various combinations of a plurality of color materials to reproduce a desired color, the combination of color materials is determined based on the evaluation value of graininess and of the size (or extent) of the reproducible color region. This technique is already in use.
Upon printing, a smallest bronzing appearance amount is not simply regarded as best. Bronzing with a different tint appears due to the difference of the reproduced chromaticity, thereby deteriorating the image quality of a printed image including a plurality of colors. That is, suppression of uneven bronzing is also regarded as important. Therefore, as in the above-described Japanese Patent Laid-Open No. 2001-138555, a color separation method has been proposed which considers suppression of bronzing as well as image quality determination factors such as graininess.
However, since the above-described method depends on visual evaluation by an image observer, a person must view and evaluate a number of images, resulting in a considerable cost. In addition, stable evaluation values cannot be obtained due to a change in the physical condition of the image observer and a change of image observers. Although bronzing can be suppressed to some extent by the above method, uneven bronzing which seriously causes image quality deterioration cannot be suppressed.
The feature of the present invention is to solve the problems of the above-described conventional techniques.
According to the features of the present invention, optimal color separation can be performed in accordance with printing conditions and image data to be printed.